Optoelectronic devices rely on the optical and electronic properties of materials to either produce or detect electromagnetic radiation electronically or to generate electricity from ambient electromagnetic radiation. Photosensitive optoelectronic devices convert electromagnetic radiation into electricity. Photovoltaic (PV) devices or solar cells, which are a type of photosensitive optoelectronic device, are specifically used to generate electrical power. PV devices, which may generate electrical power from light sources other than sunlight, are used to drive power consuming loads to provide, for example, lighting, heating, or to operate electronic equipment such as computers or remote monitoring or communications equipment. These power generation applications also often involve the charging of batteries or other energy storage devices so that equipment operation may continue when direct illumination from the sun or other ambient light sources is not available. As used herein the term “resistive load” refers to any power consuming or storing device, equipment or system. Another type of photosensitive optoelectronic device is a photoconductor cell. In this function, signal detection circuitry monitors the resistance of the device to detect changes due to the absorption of light. Another type of photosensitive optoelectronic device is a photodetector. In operation a photodetector has a voltage applied and a current detecting circuit measures the current generated when the photodetector is exposed to electromagnetic radiation. A detecting circuit as described herein is capable of providing a bias voltage to a photodetector and measuring the electronic response of the photodetector to ambient electromagnetic radiation. These three classes of photosensitive optoelectronic devices may be characterized according to whether a rectifying junction as defined below is present and also according to whether the device is operated with an external applied voltage, also known as a bias or bias voltage. A photoconductor cell does not have a rectifying junction and is normally operated with a bias. A PV device has at least one rectifying junction and is operated with no bias. A photodetector has at least one rectifying junction and is usually but not always operated with a bias.
Optoelectronic devices that make use of organic materials are becoming increasingly desirable for a number of reasons. Many of the materials used to make such devices are relatively inexpensive and conducive to low-cost, continuous manufacturing processes using low temperature vacuum deposition or solution processing techniques, so organic optoelectronic devices have the potential for cost advantages over inorganic devices. In addition, the inherent properties of organic materials, such as their flexibility, may make them well suited for particular applications such as fabrication on a flexible substrate or deposition on non-planar/conformal substrates. Examples of organic optoelectronic devices include organic light emitting devices (OLEDs), organic phototransistors, organic photovoltaic cells (OPV cells), and organic photodetectors. For OLEDs, the organic materials may have performance advantages over conventional materials. For example, the wavelength at which an organic emissive layer emits light may generally be readily tuned with appropriate dopants.
As used herein, the term “organic” includes polymeric materials as well as small molecule organic materials that may be used to fabricate organic optoelectronic devices. “Small molecule” refers to any organic material that is not a polymer, and “small molecules” may actually be quite large. Small molecules may include repeat units in some circumstances. For example, using a long chain alkyl group as a substituent does not remove a molecule from the “small molecule” class. Small molecules may also be incorporated into polymers, for example as a pendent group on a polymer backbone or as a part of the backbone. Small molecules may also serve as the core moiety of a dendrimer, which consists of a series of chemical shells built on the core moiety. The core moiety of a dendrimer may be a fluorescent or phosphorescent small molecule emitter. A dendrimer may be a “small molecule,” and it is believed that all dendrimers currently used in the field of OLEDs are small molecules.
OLEDs make use of thin organic films that emit light when voltage is applied across the device. OLEDs are becoming an increasingly interesting technology for use in applications such as flat panel displays, illumination, and backlighting. Several OLED materials and configurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and 5,707,745, which are incorporated herein by reference in their entirety.
OLED devices are generally (but not always) intended to emit light through at least one of the electrodes, and one or more transparent electrodes may be useful in an organic optoelectronic devices. For example, a transparent electrode material, such as indium tin oxide (ITO), may be used as the bottom electrode. A transparent top electrode, such as disclosed in U.S. Pat. Nos. 5,703,436 and 5,707,745, which are incorporated by reference in their entireties, may also be used. For a device intended to emit light only through the bottom electrode, the top electrode does not need to be transparent, and may be comprised of a thick and reflective metal layer having a high electrical conductivity. Similarly, for a device intended to emit light only through the top electrode, the bottom electrode may be opaque and/or reflective. Where an electrode does not need to be transparent, using a thicker layer may provide better conductivity, and using a reflective electrode may increase the amount of light emitted through the other electrode, by reflecting light back towards the transparent electrode. Fully transparent devices may also be fabricated, where both electrodes are transparent. Side emitting OLEDs may also be fabricated, and one or both electrodes may be opaque or reflective in such devices.
As used herein, “top” means furthest away from the substrate, while “bottom” means closest to the substrate. For example, for a device having two electrodes, the bottom electrode is the electrode closest to the substrate, and is generally the first electrode fabricated. The bottom electrode has two surfaces, a bottom surface closest to the substrate, and a top surface further away from the substrate. Where a first layer is described as “disposed over” a second layer, the first layer is disposed further away from substrate. There may be other layers between the first and second layer, unless it is specified that the first layer is “in physical contact with” the second layer. For example, a cathode may be described as “disposed over” an anode, even though there are various organic layers in between.
Photovoltaic (PV) devices, for example, solar cells, are used to generate electrical power from ambient light. PV devices are used to drive power consuming loads to provide, for example, lighting, heating, or to operate electronic equipment such as computers or remote monitoring or communications equipment. These power generation applications often involve the charging of batteries or other energy storage devices so that equipment operation may continue when direct illumination from the sun or other ambient light sources is not available. PV cells are characterized by the efficiency with which they can convert incident solar power to useful electric power. Devices utilizing crystalline or amorphous silicon dominate commercial applications, and some have achieved efficiencies of 23% or greater. However, efficient crystalline-based devices, especially of large surface area, are difficult and expensive to produce due to the cost and difficulty inherent in producing large crystals without significant efficiency-degrading defects or losses associated with their fragility. On the other hand, high efficiency amorphous silicon devices suffer from problems with stability. Present commercially available amorphous silicon modules have stabilized efficiencies between 4 and 8%. More recent efforts have focused on the use of organic photovoltaic cells to achieve acceptable photovoltaic conversion efficiencies, lower production costs and additional product features like flexibility and semi-transparency.
Traditionally, photosensitive optoelectronic devices have been constructed of a number of inorganic semiconductors, e.g., crystalline, polycrystalline and amorphous silicon, gallium arsenide, cadmium telluride and others. Herein the term “semiconductor” denotes materials which can conduct electricity when charge carriers are induced by thermal or electromagnetic excitation. The term “photoconductive” generally relates to the process in which electromagnetic radiation energy is absorbed and thereby converted to excitation energy of electric charge carriers so that the carriers can conduct, i.e., transport, electric charge in a material. The terms “photoconductor” and “photoconductive material” are used herein to refer to semiconductor materials which are chosen for their property of absorbing electromagnetic radiation to generate electric charge carriers.
PV devices include a p-n junction that enable the device to convert incident radiation into electrical current. Examples of PV devices include solar cells and photodiodes. Solar cells are designed to generate power. Solar cells are designed to generate power. Solar cells generally do not have an external voltage source, but are used to generate a current at a voltage created by the inherent properties of the organic materials used. Photodiodes include a p-n junction, and are generally provided with an external bias and may be used as a photodetector. PV devices may also include at least one transparent electrode to allow incident radiation to be absorbed by the device. Several PV device materials and configurations are described in U.S. Pat. Nos. 6,657,378, 6,580,027, and 6,352,777, which are incorporated herein by reference in their entirety.
Dielectric multilayer films may be used to manipulate the properties of electromagnetic radiation as it passes through the films. The term “dielectric” generally relates to any material that is a relatively poor conductor of electricity, while being capable of efficiently maintaining an electric field. Traditionally, a substance having a relatively low conductivity (compared to that of a metal), such as a ceramic, glass, plastic, or even air may be used as a dielectric. In some substances, such as semiconductors, conductivity increases with temperature. A dielectric material may be discussed with respect to its permittivity (or dielectric function), which is a measure of the frequency-dependent response of a material to excitation by an electric field. In mathematical notation, permittivity is written in complex notation in order to take wave absorption into account, therefore, it includes a real component relating to the polarization of the wave and an imaginary component relating to the absorption of the wave. The index of refraction of a material may also be expressed using complex notation, where the real component relates to the phase velocity of the wave and the imaginary component relates to the absorption coefficient (also called the attenuation index or extinction coefficient). Maxwell's equations give a basic relationship between the permittivity of a material and its index of refraction, therefore, the index of refraction may be described in terms of the permittivity.
A great deal of information about the interaction of an optical wave with a semiconductor material, and a useful quantification of the behavior, may be expressed in terms of either of these sets of parameters. Using the Kramers-Krönig relations, if either the real or the imaginary part of a frequency dependent quantity is known for all frequencies, the other may be calculated. Accordingly, if a full absorption spectrum is known, the index of refraction may be calculated for any frequency. Therefore, these relations are often used to derive a representation of the index of refraction from absorption or reflectance data obtained by experiment. The Kramers-Krönig relations also give rise to expressions of relationship between the reflectance and the phase difference between the electric fields of the incident and reflected waves.